The present invention relates to a two-stage process effective in converting C.sub.2 -C.sub.5 olefinic hydrocarbons into aromatic hydrocarbons via hydrogenation and dehydrocyclodimerization, respectively.
Dehydrocyclodimerization is a reaction where reactants comprising paraffins and olefins, containing from 2 to 5 carbon atoms per molecule, are reacted over a catalyst to produce primarily aromatics with H.sub.2 and light ends as by-products. This process is quite different from the more conventional reforming or dehydrocyclization process where C.sub.6 and higher carbon number reactants, primarily paraffins and naphthenes, are converted to aromatics. These aromatics contain the same or less number of carbon atoms per molecule versus the reactants from which they are formed, indicating the absence of dimerization reactions. In contrast, the dehydrocyclodimerization reaction results in an aromatic product that always contains more carbon atoms per molecule than the C.sub.2 -C.sub.5 reactants, thus indicating that the dimerizing reaction is a primary step in the dehydrocyclodimerization process. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 260.degree. C. using dual functional catalysts containing acidic and dehydrogenation components. These catalysts include acidic amorphous aluminas which contain metal promoters. Recently, crystalline aluminosilicates have been successfully employed as catalyst components for the dehydrocyclodimerization reaction. Crystalline aluminosilicates, generally referred to as zeolites, may be represented by the empirical formula: EQU M.sub.2/n.A1.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
in which n is the valence of M which is generally an element of Group I or II, in particular, sodium, potassium, magnesium, calcium, strontium, or barium and x is generally equal to or greater than 2. Zeolites have skeletal structures which are made up of three-dimensional networks of SiO.sub.4 and AlO.sub.4 tetrahedra, corner linked to each other by shared oxygen atoms. The greater the proportion of the SiO.sub.4 species to the AlO.sub.4 species, the better suited the zeolite is for use as a component in dehydrocyclodimerization catalysts. Such zeolites include mordenite and the ZSM variety. In addition to the zeolite component, certain metal promotes and inorganic oxide matrices have been included in dehydrocyclodimerization catalyst formulations. Examples of inorganic oxides include silica, alumina, and mixtures thereof. Metal promoters such as Group VIII or Group III metals of the Periodic Table have been used to provide the dehydrogenation functionality. The acidic function can be supplied by the inorganic oxide matrix, the zeolite, or both.
The use of olefins as a feedstock to a dehydrocyclodimerization reaction zone has been found to result in a lower aromatic selectivity and increased catalyst deactivation (coking) in comparison to a paraffinic feedstock. The rapid deactivation is believed to be caused by excessive carbon formation (coking) on the catalyst surface. This coking tendency makes it necessary to frequently perform costly and time-consuming catalyst regenerations. Reducing catalyst coking tendencies and thereby increasing catalyst life is a particular object to which this application is directed.